Paper having improved characteristics and its preparation



United States Patent 3,015,537 PAPER HAVING IMPROVED CHARACTERISTICS AND ITS PREPARATION Glen G. Gray and Joseph T. Leone, Rochester, N.Y., as-

signors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Apr. 26, 1956, Ser. No. 580,722

5 Claims. (Cl. 8-121) This invention relates to paper having unusual properties, and the process of making that paper.

Various methods of treating paper have been suggested to prepare a paper having permanence properties when that paper has been subjected to wetting and redrying. Regular Waterleaf paper upon wetting and redrying exhibits properties which render that paper disadvantageous for many purposes, some of those properties after wetting and redrying being dimensional change, shrinkage, loss of surface characteristics, crinklingfmottling, and the like. Also paper when wet loses its rigidity and wet strength and absorbs amounts of water to an undesirable extent. Papers have been prepared in which melamine or urea formaldehyde resins have been incorporated therein to overcome the loss of strength upon Wetting. However, those papers still have disadvantages when they are subjected to wetting and redrying. The use of wet strength resins has increased the cost of preparing paper. Also, papers having wet strength resins are often found to be undesirable for use for photographic purposes.

To improve the properties of papers sizing materials have been applied to the paper either as engine sizing, in the form of tub sizing, or as an application to the surface of the paper. Some of the materials which have been employed in this connection are stearic acid, starch, gelatin, rosin, latices and also melamine-formaldehyde and urea-formaldehyde synthetic materials. 'Ihose applications to the paper have helped to some extent but nevertheless there has been considerable room for improvement in the properties of the resulting product.

When machine-made paper is calendered on the papermaking machine or as a separate operation, a paper having a smooth glossy surface is obtained. The Wetting of this paper, as prepared by most procedures heretofore used, may cause loss of smoothness and gloss and loss of dimensional characteristics after the paper has been again dried. While the paper is wet it lacks rigidity, making for difficult handling. The paper may be stained as a result of its high water absorption.

In the case of some papers, coatings of an aqueous nature are applied thereto which may impart curl characteristics thereto which are undesirable. Also the coatings which are applied being aqueous in nature result in a wetting and redrying of the paper in the coating operations which may cause release of the effect obtained by calendering. These changes in the paper are evidenced by increased thickness thereof, increased surface roughness and reduced gloss. For instance, differential swelling of the surface fibers of paper during coating operations can completely destroy the smooth glossy surface obtained thereon in calendering. Processing of the paper results in complete wetting of the paper. Wetting in processing causes differential swelling of the surface fibers thereof and subsequent drying results in increased surface roughness, loss of gloss, change in dimensions, etc.

One object of our invention is to provide a paper which has good resistance to the effects of water thereon and in addition exhibits substantially permanent surface characteristics. Another object of our invention is to provide a paper having a smooth glossy surface which Will not become appreciably roughened by contacting the paper with water. A further object of our invention is to provide a paper which when coated with an aqueous com- ICC position exhibits characteristics which are desirable as regards dimensional stability, rigidity, smoothness and gloss characteristics. A still further object of our invention is to provide a method of making paper base of high wet strength Without the necessity of using formaldehyde-forming resins which heretofore have been considered desirable in the making of high wet strength paper but which may be undesirable for other reasons. Other objects of our invention will appear herein.

The paper of our invention exhibits properties superior to regular paper in many respects. For instance, the paper of our invention exhibits a dimensional change of less than 1% laterally when that paper is subjected to a wetting with Water for two minutes in a Schopper expansion tester. The paper of our invention exhibits a shrinkage of less than 1% laterally when that paper is soaked in water, dried and then reconditioned. It exhibits a change of less than 15% in a vertical direction upon wetting, and wetting, drying and reconditioning showed a change of less than 5%. Its water absorption by a oneminute Cobb size test is less than .15 gram per 100 I square centimeters. The paper of our invention when subjected to the Valley size penetration test exceeds 2,000 seconds, the maximum value to which that test was carried. This paper exhibits a rigidty when soaked in water at least twice that of the original waterleaf paper as measured upon a Taber V-S stiffness tester. The dry rigidity of this paper is greater than that of the original waterleaf paper especially in the cross direction.

When paper in accordance with our invention is wet and dried, it is found that the smoothness thereof is not substantially changed as is indicated by measuring the redried paper upon a Surfindicator indicating that the roughness thereof does not exceed microinches. This is in distinction to regular types of wet strength paper in which wetting and redrying causes roughening of the surface of the paper. Also wetting and redrying of paper in accordance with our invention does not decrease the reflectance thereof to the extent encountered with regular grades of paper when those papers are wetted and redried. When paper in accordance with our invention is prepared from high alpha cellulose type pulps, the brightness thereof is not decreased after exposure of those papers to ultraviolet light as measured by brightness determinations upon papers which have been so treated. Papers which have been previously made from high alpha cellulose pulps such as are used for the better and more stable grades of paper employed for photographic purposes show some decrease in brightness when treated in ultraviolet light. Whereas papers made with lower alpha cellulose pulps heretofore have decreased appreciably in brightness when exposed toultraviolet light, those made in accordance with our invention from lower alpha cellulose pulps (such as having an alpha cellulose content of -88%) have been found to experience no decrease of brightness after exposure to ultraviolet light of comparable intensities and exposure times.

Paper in accordance with our invention is prepared by the esterification of paper in sheet form to impart an acyl content of 10-30% thereto. This esterification can be accomplished by treating the paper with a process which imparts lower fatty acid radicals to the cellulose of the paper which process would include the use of a bath comprising a lower fatty acid anhydride and an esterification catalyst which bath preferably also contains a solvent. The lower fatty acid anhydride may be acetic anhydride, propionic anhydride, butyric anhydride or isobutyric anhydride or the esterification mixture may be made up of a lower fatty acid such as acetic, propionic or butyric acid and an impelling anhydride such as chloracetic anhydride or an alkoxy acetic anhydride. The esterification catalyst employed may be perchloric acid,

zinc chloride, phosphoric acid or even sulfuric acid or it may be a basic catalyst such as an alkali metal acylate, for instance, potassium acetate, potassium butyrate or the like. When solvent is employed it may be a lower fatty acid or an inert organic liquid such as methylene chloride, ethylene chloride, carbon tetrachloride or the like. The esterification of the paper sheet may be carried out in any one of various ways such as by immersing it in the esterification bath for the time necessary to impart the desired acyl content at the temperature which is employed. The paper sheet is conveniently first treated with a pretreatment liquid to swell the cellulose making it more susceptible to the action of the acid anhydride thereon. The preferred method of preparing acylated paper is to subject the paper to a pull-through of the materials which act thereon. For instance, the paper, for example, would be subjected to a pull-through of water and the water would be removed from the sheet such as by a pull-through of a lower fatty acid. The paper sheet may then be subjected to a pull-through of an esterification bath in which an acid anhydride may cause the chemical combination of the cellulose of the paper sheet with lower fatty acid groups. After the cellulose is esterified to the desired extent, it is washed such as by a pullthrough of water followed by drying. This drying is conveniently carried out by pressure drying which assists in obtaining a sheet in accordance with our invention. There is thus obtained paper sheet having a content of acyl therein within the range of -30% and preferably within the range of -20% of acetyl, propionyl, butyryl or isobutyryl. There is no advantage is exceeding a 26% acyl content particularly where the acyl is acetyl while on the other hand, the desired properties are less pronounced as the acetyl content decreases below 15%.

After the paper has been acylated, it may be submitted to pressure drying as the paper comes from the washing operation subsequent to the acylation or it may be dried in a conventional manner, rewetted and then subjected to the pressure drying operation. The acylated paper which is treated should contain a liquid such as water or acetone in the amount of at least 12% such as 12-50% of the paper mass. The paper obtained from the acylation after Washing free of the materials employed in that operation will ordinarily have a high water content and can be employed directly in the pressure drying operation. If, however, the acylated paper has been previously dried, a liquid content should be imparted thereto such as by soaking in water-miscible liquid such as water, acetone, alcohol or the like, or spraying the paper with liquid in the desired amount. The paper in this condition is then subjected to heat and pressure while at least one surface of the paper is in contact with a polished metal surface. This operation can be conducted by passing the paper between polished metal rolls or over a polished metal roll having a pressure roll bearing thereon or the paper can be subjected to pressure with a flat polished metal surface which has been polished. The temperature employed should be at least 200 F. and preferably 300 F. or more at a mechanical pressure from 300 to 1,000 or more p.s.i. The paper is reduced to a moisture content of not more than 5% by this treatment. Ordinarily when heated metal rolls are employed, more than one passage is desirable for the purpose of reducing the moisture content to 5% or less. The paper obtained has resistance to the effects of moisture and has a permanently smooth surface, thus giving a product having characteristics advantageous for various usages.

The paper obtained by the procedure described herein may, if desired, be coated with various aqueous coating compositions such as clay colors or like coating compositions. The resulting product has a smoothness and glossiness which is retained.

Paper in accordance with our invention exhibits a dry stiffness greater than that of the original waterleaf paper from which it was prepared, especially in the cross direction. This value is determined on a Taber V-S stifiness tester, as supplied by the Taber Instrument Corporation, North Tonawanda, New York. This test is described in Instrumentation Study No. 14 of the Institute of Paper Chemistry, dated May 27, 1937. In the results recorded of these tests L designates the values obtained in the machine direction and C the values obtained in the cross direction of the paper.

Paper prepared in accordance with our invention ex hibits a wet stiffness which is at least twice that of the waterleaf paper from which it was prepared. The Wet stiffness of paper is determined by soaking the paper samples for three minutes in Water at 7375 F. and then determining the stiffness values on the Taber V-5 stiffness tester using the sensitive 0-10 range.

The dimensional change, in water, of paper prepared in accordance with our invention is less than 1% laterally in the two-minute test, determined by means of a Schopper expansion tester. In this test, a strip of paper six inches by fifteen millimeters is cut from the crossdirection of the paper used. This sample is fastened by the clamps of that device using care that it is not put under tension or buckled. The sample is then immersed in a jar of water for two minutes at the end of which time the scale reading is noted. The scale reading gives the wet expansion in percent.

Paper in accordance with our invention exhibits a shrinkage of less than 1% laterally. This value is determined after first conditioning the paper in an atmosphere of 50% relative humidity. There is then marked off on the paper a given length. The paper is then soaked in water and then dried. The paper is reconditioned at 50% relative humidity. The length on the sample which has been marked off is then measured and the percent of shrinkage can be obtained therefrom.

The paper in accordance with our invention exhibits a change when wetted of less than 15% in the vertical direction. This value is obtained by using a caliper on the the paper following which the paper is soaked thoroughly in water and a caliper reading is again taken. The difference in the two caliper readings indicates the change in the vertical direction of the paper upon wetting. When the paper is redried the difference in caliper readings indicates a change of less than 5% from the original.

The water absorption of paper in accordance with our invention by the one-minute Cobb size test is less than .15 gram per 100 square centimeters. In this test water is allowed to contact a fixed area of paper for a definite time and the amount absorbed is determined by the gain in weight of the sample (see TAPPI method T441m). When paper according to our invention is tested in the Valley penetration test, the value obtained exceeds 2,000 seconds which is the maximum reading taken in that test. The testing instrument used was the Valley size tester and in these tests a two-inch square sample of the paper is placed between the two cells of the machine. The set screws are tightened up, the lever at the back is raised and the time is checked by means of a time clock. The results are recorded as the number of seconds taken for the current to reach the required number of milliamperes. The solution used in the Valley size tester is as follows:

Distilled water cc 516 Sodium chloride gr 24 Glycerin cc 60 The permanence of the surface characteristics of paper is often determined by subjecting the paper for two hours to ultraviolet light using mercury arc lamp GEUA2- 250W using a temperature of ll0 F. inside the enclosure. TAPPI Standard T-452 describes the method of determining the percent reflectance. Paper in accordance with our invention prepared from pulps having lower alpha cellulose contents than have been previously used for making papers of high permanence to ultraviolet light have been found to retain their character istics thereunder when so tested. For instance, papers in accordance with our invention have been prepared from wood pulps having alpha cellulose contents of 8589%. The papers which have been obtained by our invention when tested in this ultraviolet test have retained a degree of reflectance as great as that of the original papers before subjecting to ultraviolet light. Previously it has been necessary to prepare papers from pulps having a much higher alpha cellulose content in order to obtain a comparable resistance to the effect of ultraviolet light. Papers in accordane with our invention which have been prepared from high alpha cellulose pulps (90-95% alpha cellulose content) exhibit a brightness of at least 90% reflectance after exposure of those papers to ultraviolet light in a GE. spectrophotometer at 456 millimicrons wavelength. In this testing device the overall length of the ultraviolet tube used is 6% inches with a 12 inch distance between the tube and the drum holding the samples of paper which are subjected to the ultraviolet light.

Paper in accordance with our invention also exhibits resistance to roughening upon wetting of that paper and drying. When this paper is wet and dried and the resulting paper is tested in a Surfindicator as made by the Brush Electronics Company, Cleveland, Ohio, the roughness of the paper does not exceed 85 microinches which constitutes a considerable improvement over papers which have previously been prepared for use in situations where wetting may occur. For instance, whereas a paper in accordance with our invention will after wetting and redrying give a test on the Surfindicator of no greater than 85 microinches of roughness, a good standard grade of paper will exhibit a roughness of 120 or more microinches.

Paper in accordance with. our invention having high initial brightness when wetted, redried and then tested with a Gardner gloss meter exhibits at least 25 reflectance units. Thus paper, in accordance with our invention retains its gloss even though subjected to the effects of water.

The following examples illustrate our invention:

Example 1 The paper employed was made from wood pulp consisting of 50 percent hard wood sulfite pulp and 50 percent of soft wood pulp. The pulp was dispersed in water and was adjusted in the paper-making process with aluminum chloride to an acidity of 3-5 milliequivalents per liter. Three percent melamine-formaldehyde resin (based on cellulose) was added to the pulp mass to impart wet strength. Paper was prepared therefrom by coating out on the wire in a conventional paper-making machine, following which the paper was dried on steam heated drums. Sheets of the paper (8" x were acetylated by the following procedure:

(1) The sheets were pretreated by pulling through the paper 500 ml. of water, then 500 ml. of glacial acetic acid and finally 500 ml. of acetic anhydride containing 5 percent phosphoric acid. The temperature of the reagents used was 73 F. and the contact times used in each pull-through operation was -20 seconds using a vacuum of 2.1 cm. of mercury.

The sheets were then removed and placed in a bath consisting of acetic anhydride containing 5 percent of phosphoric acid (85 percent) at a temperature of 73 F. After keeping the sheets for 1 hour in this bath, they were removed and placed in position for a pull-through of the following:

A. Three washes with 200-300 ml. of alcohol, three washes with 200300 ml. of water and where indicated two washes with 200-300 ml. of acetone.

The following samples were prepared:

A. Untreated comparison.

B. Soaked in water, then pressure dried in a hydraulic press.

C. Acetylated, water final rinse, dried in a commercial dryer (Pako).

D. Acetylated, acetone final rinse, dried in Pako dryer.

E. Acetylated, Water final rinse, pressure dried in hydraulic press.

F. Acetylated, acetone hydraulic press.

G. Acetylated, water final rinse, dried in Pako dryer, then pressed in hydraulic press.

H. Acetylated, water final rinse, dried 'in Pako dryer, then rewetted with water and dried in hydraulic press.

The hydraulic press used was a 30-ton. press having polished metal contact surfaces. The Pako dryer corn prises a drum having a smooth unpolished metal surface over which paper is led for a distance of about 5 feet at a temperature of 250 F. The paper is held onto thedrurn by means of a belt using only sufficient pressure toassure contact between the paper and the drum. The paper sam ples that were given a final rinse with water were run through wringer rolls to remove excess surface water. The acetone rinse samples were not put through the wringer. The wet sheets having approximately 30 percent water or acetone were, when the hydraulic press was used, placed between polished chrome plates and were .subjected to 700 p.s.i. at 300 F. for 10 minutes. The pressure was relieved periodically to allow, vaporto ,escape.

After the treatment was completed,th'esheets werecu't in half, one series of sheets being identified as A to H the other series as A to H The paper sheets in the series A to H were held at 48 percent relative humidity. The sheets in the series A to H were soaked in water at 75 F. for 30 minutes after which they were air dried at 75 F. and 48 percent R.H. The samples were then tested for visual gloss, meter gloss, surface roughness, cross expanfinal rinse, pressure dried in sion 1n water, dry bursting strength (or Mullen) and wet strength. Results were as follows:

Gloss Cross Identity Cali- Difier- Rough- Exp, Mul- Wet per ence ness Perlen Stg.

25 45 cent 4. 7 1. 42 1. 20 0. 22 65 1. 65 22 9% 5. 5 1. 42 1. 24 0.18 83 1. 50 20 8% 4.1 1. 45 1.15 0. 30 50 1. 30 20 8% 4. 4 1. 45 1. 20 0. 25 64 1. 25 23 8% 7. 3 1. 44 1. 27 0.17 110 0. s0 is 9% 7. 5 1. 43 1.28 0.15 115 0. 75 18 10 7. 5 1. 43 1.28 0. 15 113 0. 60 21 11% 7. 6 1. 44 1. 26 0. 16 127 0. 60 21 12 5. 1 1. 46 1. 14 0. 32 65 0. 45 20 10% 5.3 1.42 1.20 0.22 82 0.50 is 10 5. 5 1. 43 1. 1s 0. 25 0. 50 19 11% 5. 5 1. 42 1. 20 0. 22 78 0.45 21 9% 5. 3 1. 42 1. 21 0. 21 93 0.85 17 s 6. 0 1. 41 1. 23 0. 18 83 0. 17 8 4. 3 1. 43 1.14 0. 31 65 0. 55 18 7% 4. 9 1. 41 1. 19 0. 22 78 0. 88 15 6% 1 Visual ratings of gloss grouped the samples as follows: Highest gloss B1, E1, F1, H1 Next highest group A1, G1, E2, F2, H7 Some gloss B1 Virtually no gloss C1, D1, A2, C2, Dz, G2.

The gloss ratings are based on the intensities of reflected light read in density units (logarithm). The difference between the rating at 25 and 45 is the logarithm of the ratio of specular to diffuse reflection. A difference of 0.02 is considered significant.

Roughness was measured in microinches with a Surfiindicator. Instrument settings were: range 300 microinches and cutofi 0.030 inch.

Cross expansion was measured with a Louis Schopper' expansimeter.

1 The Mullen or bursting strength wa measured on a B. F. Perkins tester, TAPPI Standard T-403M-4l. The wet strength was determined by means of a Minden wet strength tester.

Example 2 The paper used here was paper having a base weight of 25 /2 pounds per 1,000 square feet made from a mixture of several pulps. The pulp suspension was adjusted with aluminum chloride to an acidity of between 3 and 5 milliequivalents per liter. One percent melamine formaldehyde resin (based on dry cellulose) was added to impart wet strength. All the paper sheets were treated as follows:-

'These sheets (8" x 10'') were pretreated by pulling through the paper 500 ml. of distilled water, then 500 m1. of glacial acetic acid. The temperature of the water and the acid was 73 F. and the contact time for each was to seconds under a vacuum of 2.1 cm. of mercury.

The sheets were then acetylated by pulling through the paper 500 ml. of acetic anhydride containing 5 percent phosphoric acid (85 percent) at a temperature of 230 F. the contact time with the anhydride was 15 to 20 seconds under a vacuum of 2.1 cm. of mercury.

The reaction was quenched almost immediately by pulling through the sheets the following liquid at 73 F.; three washes with 200-300 ml. of alcohol. Three washes with 200 300 ml. of water and where indicated, two washes with 200-300 ml. of acetone. The identities and descriptions of the treatments given are as follows:

A. Untreated'comparison.

B. Water treated comparison, Pako dried, then pressure dried in a hydraulic press.

C. Water treated comparison, pressure dried while wet in a hydraulic press. 7

D. Acetylated, water final rinse, Pako dried.

E. Acetylated, acetone final rinse, Pako dried.

F. Acetylated, water final rinse, Pakodried, then rewetted and pressure dried in a hydraulic press.

G. Acetylated, water final rinse, pressure dried in a hydraulic press while wet.

The pressure drying was the same as described in the preceding example. The starting moisture content in each case was approximately 50 percent. In the pressure drying the paper was pressed for about 10 minutes with relief periodically to permit escape of vapor. The sheets obtained were cut in half and were identified as two groups A to G and A to G The samples were treated as described in the preceding example and were tested. These test results were as follows:

Gloss Cross Identity Cali- Difi'er- Rough- Exp Mul- Wet per ence ness Perlen Stg.

45 cent 6. 3 1. 44 1. 25 0.19 81 2.15 42 12% 7.3 1. 43 1. 25 0. 18 101 2. 50 42 13% 7. 2 l. 46 1. 29 0. 17 105 2. 20 41 12 7. 4 1. 44 1. 27 0. 17 103 2. 40 43 13 4. 9 1. 47 1. 20 0.27 74 1.80 47 11 5.0 1. 44 1. 19 0. 25 78 2.05 46 12% 9. 6 1.45 1. 29 0. 16 134 1. 15 25 10% 9.8 1v 46 1. 30 0.16 141 1. 10 28 9 9. 8 1. 46 1. 20 0. 17 132 0. 85 34 17% 10.0 1. 46 1. 29 0.17 136 0. 85 34 14% 5. 5 1. 47 1.07 0. 40 49 0.85 35 15 6.0 1. 46 1. 14 0.32 62 0. 90 39 17% 5. 5 1. 46 1. 07 O. 39 65 0. 65 40 18% 5. 5 1. 1. 16 0.29 65 0. 75 38 9 Preparation of partially acetylated paper may take place in various manners; for instance, in one procedure sheets (8" x 10") of paper were treated by utilizing a vacuum of about 20 inches of water beneath the sheet and pulling through the paper the following:

(1) 250 ml. of tap water at room temperature (10 seconds).

2(2) 250 ml. of glacial acetic acid at room temperature (10 seconds).

(3) 250 ml. of acetic acid-acetic anhydride mixture (3-l) containing 0.3 percent perchloric acid at 143 F. (15 seconds).

(4) 250 ml. of glacial acetic acid at room temperature 10 seconds).

(5) Tap water at room temperature (1530 seconds).

The paper was then pressure dried and was found to be suitable for use in preparing a photographic paper base in accordance with our invention.

Other esterification catalysts such as sulfuric acid, phosphoric acid, methane sulfonic acid, zinc chloride or the like may be convenient for use in preparing partially acetylated paper. The rate of esterification can be increased by increasing catalyst concentration and/or anhydride content and/ or temperature.

In another case, partially acetylated paper was prepared by placing rolls of paper on a stainless steel support and treating by dipping and agitating in a series of glass cylinders as follows:

(1) Pretreat paper in water containing 2 percent phosphoric acid by volume (5 minutes).

' (2) Dewater with glacial acetic (5 minutes). Dewater a second time (5 minutes).

(3) React paper with acetic anhydride containing 5 percent phosphoric acid by volume. Maintain the temperature between and F. (1-2 hours).

(4) Wash with benzene (5 minutes).

(5) Wash with alcohol (5 minutes).

(6) Washing with running water (30-60 minutes). Then pressure dry.

Example 3 Several l0-inch webs of esterified paper which had been esterified by the procedure described in column 8, lines 7-21, supra, were calendered through one nip of a super calender having a highly polished metal surface, at a temperature of greater than 200 F. and at a pressure of greater than 300 p.s.i., the paper initially having a moisture content of 25%. The treatment of the paper was such as to reduce the moisture content thereof to a moisture value of less than 5%.

The physical properties of the esterified-pressu-re dried paper were compared with those of regular paper both of a weight of 27 pounds per thousand square feet. A

' comparison of the characteristics of the two papers is as follows:

The percentages of cross expansion and swelling of the papers in the table are based on the change in those values when the papers are changed from the dry to the wet state. The increase in bulk is the increase in caliper between the dry state and, after the paper is wetted, and then dried, the redried state.

Papers in accordance with our invention are resistant to the effects of ultraviolet light in comparison with regular paper and in some cases the reflectance of the paper after treating with ultraviolet light is even superior to that which the paper had before treatment. This holds true with partially acylated paper as described herein both in the case of paper which has been pressure dried and paper which has been nonpressure dried. The reflectance of the paper in every case was measured with a G.E. spectrophotometer before and after its exposure in an ultraviolet light chamber, this testing of paper being carried out as described supra.

In the case of a paper prepared from a 50-50 blend of soft wood pulp having an alpha cellulose content of 88% and a hard wood pulp having an alpha cellulose content of 89%, the paper obtained had a reflectance of 90.3% and after exposure to ultraviolet light of 80.1%. This same paper which had been acetylated, such as to impart an acetyl content thereto of approximately 20%, had a reflectance before submitting to ultraviolet light of 90.6% and after treating with ultraviolet light had a reflectance of approximately 91.5%. To avoid loss of reflectance with regular types of paper, it has been previously necessary to use Wood pulp having a high alpha cellulose content. For instance, with paper made from soft wood high alpha cellulose pulp (94% alpha cellulose content) made in the manner ordinarily used for making photographic paper base which included tub sizing and engine sizing thereof, the paper before subjecting to ultraviolet treatment had a reflectance of 89.7% and after ultraviolet treatment of 86.9%.

Throughout the specification the term pressure drying is used to refer to the treatment of partially acylated paper containing at least 12% of water-miscible liquid at an elevated temperature and pressure (at least 200 F. and at least 300 p.s.i.) against a polished metal surface.

It is to be understood that the use of wet-strength paper originally for the preparation of paper in accordance with our invention has been to facilitate handling. Where the paper sheet is supported while wet a straight water-leaf paper having no size of any kind can be used. The type of paper used is a choice of the individual operator for convenience in the paper preparation operation. After paper has been treated in accordance with our invention, it possesses high wet strength characteristics.

Partially acylated paper exhibits good retention of properties under severe climatic conditions. For instance, samples of acetylated paper (about 20% acetyl), high alpha waterleaf paper and good grade paper were placed in trays and were buried in the tropics for 2 weeks under a mixture of leaf mold, top soil and coarse sand (equal parts). The water content of the burying material was maintained at 27-29% and the temperature at 8283 F., the soil temperature surrounding it being 808l F. The pH of the burying material was 6.9. Upon exhuming the samples it was found that the acylated paper was white and was not decomposed to any great extent whereas the samples of good paper base and of water-leaf paper had gone to pieces or disintegrated with bad discoloration such as with patches of pink, green, purple, etc.

We claim:

1. Paper having a lower fatty acid radical content of 10-30% chemically combined therewith which paper wet with at least 12% of water-miscible liquid has been simultaneously pressed at 3001,000 p.s.i. in contact with a polished metal surface and dried at a nondeteriorating temperature of at least 200 F. to a liquid content of not more than 5% and has the following characteristics:

1. dimensional change laterally when wetted with water of less than 1%;

2. shrinkage of less than 1% laterally when soaked in water, dried and reconditioned back to original relative humidity;

3. change in a vertical direction of less than 15% when wetted with water;

4. change in a vertical direction of less than 5% when wetted with water, redried and reconditioned back to original relative humidity;

5. water absorption at 73 F. by the one-minute Cobb size test of less than .15 gram per 100 square centimeters;

6. a roughness which does not exceed 85 microinches when wetted and redried;

7. a reflectance which is not decreased by wetting and drying.

2. Paper having a lower fatty acid radical content of 30% chemically combined therewith prepared from wood pulp having an u-cellulose content of at least 85% which paper wet with at least 12% of water-miscible liquid has been simultaneously pressed at 3001,000 p.s.i. in contact with a polished metal surface and dried at a nondeteriorating temperature of at least 200 F. to a liquid content of no more than 5% and has the following characteristics:

1. dimensional change laterally when wetted with water of less than 1%;

2. shrinkage of less than 1% laterally when soaked in water, dried and reconditioned back to original relative humidity;

3. change in a vertical direction of less than 15% when wetted with water;

4. change in a vertical direction of less than 5% when wetted with water, redried and reconditioned back to original relative humidity;

5. water absorption at 73 F. by the one-minute Cobb size test of less than .15 gram per 100 square centimeters;

6. a roughness which does not exceed microinches when wetted and redried;

7. a reflectance which is not decreased by wetting and drying;

8. a brightness which is not decreased by exposure to ultraviolet light.

3. A method of preparing paper having substantially permanent surface characteristics and good resistance to the effects of moisture which comprises the steps of partially acylating the cellulose of the paper so as to impart thereto a 1030% content of lower fatty acid radicals chemically combined therewith and subjecting the paper in a condition wet with a water-miscible liquid in the amount of at least 12% simultaneously to heat and pressing between two solid surfaces at least one of which is a polished metal surface at 300 to 1,000 p.s.i. and a non-deteriorating temperature of at least 200 F. to reduce the liquid content of the paper to a value not more than 5%.

4. A method of preparing a paper having substantially permanent surface characteristics and good resistance to the effects of moisture which comprises acetylating the paper to impart thereto an acetyl content of at least 15-20% chemically combined therewith and subjecting the partially acetylated paper while wet with a watermiscible liquid to the amount of at least 12% simultaneously to heat and pressing between two solid surfaces at least one of which is a polished metal surface at 300 to 1,000 p.s.i. and a nondeteriorating temperature of at least 200 F. until the liquid content of the paper has been reduced to a value of not more than 5%.

5. A method of preparing a paper having substantially permanent surface characteristics and good resistance to the effects of moisture which comprises acylating cellulose paper to impart thereto a content of 10-30% of fatty acid radicals of 24 carbon atoms chemically combined therewith and subjecting the partially esterified paper with a water content of at least 12% simultaneously to a pressing at 3001,000 p.s.i between two solid surfaces at least one of which is a polished metal surface and a temperature of at least 200 F. to reduce the water content of the paper to not more than 5%.

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3. A METHOD OF PREPARING PAPER HAVING SUBSTANTIALLY PERMANENT SURFACE CHARACTERISTICS AND GOOD RESISTANCE TO THE EFFECTS OF MOISTURE WHICH COMPRISES THE STEPS OF PARTIALLY ACYLATING THE CELLULOSE OF THE PAPER SO AS TO IMPART THERETO A 10-30% CONTENT OF LOWER FATTY ACID RADICALS CHEMICALLY COMBINED THEREWITH AND SUBJECTING THE PAPER IN A CONDITION WET WITH A WATER-MISCIBLE LIQUID IN THE AMOUNT OF AT LEAST 12% SIMULTANEOUSLY TO HEAT AND PRESSING BETWEEN TWO SOLID SURFACE AT LEAST ONE OF WHICH IS A POLISHED METAL SURFACE AT 300 TO 1,000 P.S.I. AND A NON-DETERIORATING TEMPERATURE OF AT LEAST 200*F. TO REDUCE THE LIQUID CONTENT OF THE PAPER TO A VALUE NOT MORE THAN 5%. 